Method of foaming plastics using an alternating magnetic field

ABSTRACT

A METHOD OF PRODUCING AN ARTICLE HAVING A HOLLOW SHELL FILLED WITH AN EXPANDED FOAM. THE HOLLOW SHELL IS FORMED OF A NATERIAL OF WHICH TRANSMITS AN ALTERNATING MAGNETIC FIELD. THE HOLLOW HELL IS FILLED WITH A FOAMING AGENT COMPRISED OF A GOAMABLE LIQUID, HAVING UNIFORMLY DISPERSED THEREIN INDUCTIVELY HEATABLE SUSCEPTOR PARTICLES. THE ARTICLE IS SUBJECTED TO AN ALTERNATING MAGNETIC FIELD WHICH HEATS THE PARTICLES, EXPANDS THE FOAMABLE LIQUID, AND FILLS THE INTERIOR OF THE HOLLOW SHELL.

June 11, 1974 w, c. HELLER. JR.. ETAL 3.816.574

METHOD OF FOAMING PLASTICS USING AN ALTERNATING MAGNETIC FIELD Filedlarch 51, 1972 FOAMABLE COMPOSITION fi US. Cl. 264-45 7 Claims ABSTRACTOF THE DISCLOSURE A method of producing an article having a hollow shellfilled with an expanded foam. The hollow shell is formed of a materialof which transmits an alternating magnetic field. The hollow shell isfilled with a foaming agent comprised of a foamable liquid, havinguniformly dispersed therein inductively heatable susceptor particles.The article is subjected to an alternating magnetic field which heatsthe particles, expands the foamable liquid, and fills the interior ofthe hollow shell.

CROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part application of U.S. patent application, Ser. No.882,920, filed Dec. 8, 1969, and now abandoned.

BACKGROUND OF THE INVENTION Field of the Invention The present inventionrelates to the manufacture of articles having an expanded plastic foamcore.

Description of the Prior Art It is often desired to manufacture articlesformed of a plastic, hollow shell filled with a foamed core. Forexample, in recent years automobile safety requirements have broughtabout the installation of numerous padded elements, such as sun visorsand dash boards, in vehicles. Automotive and other seating applicationsmay also employ such articles.

In the past, articles, such as sun visors, have proven difiicult tomanufacture. The article core may be cut to size and the shellfabricated around it. However, this many be time consuming and tediousand the complete product may not have the desired finish. A precut coremay be slipped into a hollow shell only with difiiculty and the matingof the two elements is usually poor.

The poor heat transmitting characteristics of the plastic shell hasprecluded the insertion of an unfoamed core into the hollow shell to befoamed in situ. The shell prevents obtaining temperatures of themagnitude and evenness necessary for satisfactory in situ foaming.

SUMMARY OF THE PRESENT INVENTION The present invention is thus directedto an improved method for forming foamed filled articles whichalleviates the problems heretofore encountered in the production of sucharticles.

Briefly, the present invention contemplates the injection of a foamableliquid containing inductively heatable susceptor particles in the hollowshell. After injection, the article is exposed to an alternatingmagnetic field to heat the particles and expand the composition withinthe hollow shell.

In as much as the practice of the method of the present inventiongenerates heat within the shell, rather than requiring the transmissionof external heat, fully adequate and thorough foaming is obtained while,at the same time, the possibility of heat degradation of the shell,attendant United States Patent 3,816,574 Patented June 11., 1974 priorart in situ foaming processes, is completely overcome.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross sectional viewshowing a step in forming a hollow shell.

FIG. 2 shows another step in the process of the present invention, saidstep comprising injecting a foamable liquid composition into the hollowshell.

FIG. 3 is a detailed cross sectional view showing the filled hollowshell and the additional step of exposing the filled shell to analternating magnetic field.

FIG. 4 is a detailed cross sectional view showing a portion of thecompleted article manufactured by the process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to the Figures,there is shown therein a typical embodiment of the method of the presentinvention. While the article formed by the process of the presentinvention is shown in the Figures and described herein as an automobilesun 'visor, it will be appreciated that numerous other articles may beso constructed.

In the Figures, the numeral 10 indicates a hollow shell for a foam corearticle. Hollow shell 10 may be formed of a sheet of any commonlyavailable plastic material, for example, polyvinyl chloride. The plasticmay be placed in vacuum thermo-forming die 12 which forms the plasticsheet into the desired hollow shell 10. In the case of an automobile sunvisor, hollow shell 10 may comprise an envelope having one open end intowhich may be injected the foamable composition 14 described next below.

Composition 14 is formed of a foamable liquid. Such a material may beselected from available urethane liqquids. Vinyl plastisols may also beemployed. The foregoing liquids produce a resilient or semi resilientfoam. In the event a more brittle foam is desired, a liquid epoxy, orpolyester resin compound may be utilized.

A blowing agent is incorporated in the liquid of foamable composition14. This agent may be a solid granular material such as ABFA(azobisformamide) or azodicarbonamide which decomposes to produce gas ina range of from to 200 C. In the alternative, a liquid having a lowboiling point, such as Freon l1 (dichlorodifluoromethane), may be usedas the blowing agent in foamable composition 14.

There is also added to foamable composition 14 susceptor particles 16(see FIG. 3) heatable upon exposure to an alternating magnetic field.Susceptor particles 16 are incorporated in foamable composition 14 inquantities sufiicient to produce the heating action necessary to raisethe temperature of the blowing agent to the decomposition temperature.This is typical 5 to 50% by weight with respect to the foamable liquidof the composition.

The susceptor particles may be compounds comprising, at least in part,electrically nonconductive metallic oxides having thermo-hystereticproperties. The oxide compounds gamma Fe O and Fe O have been found tobe particularly useful susceptors. CrO may be used. Ferrite materialsmay also be used.

In addition to their high heat generating properties by hysteresislosses, such compounds may be reduced to extremely small size. The sizereduction is without loss of heat generating properties. The sizereduction minimizes any tendency of the susceptor particles to settleout and facilitates handling and compounding by known existing equipmentto the ends of economy and commercial production. Metallic oxidesusceptors may be reduced to submicron particle sizes, for example, .01microns. Typically, the particles might range in size from submicron to20 microns. However, since a fewer number of large particles willprovide the same particle loading as a large number of small particles,under certain circumstances, large particles may be employed to reducecoloration of the foam composition. Such large particles may have sizesranging up to 250 microns.

Foamable composition 14 so formed may be stored in tank 18 and injectedinto hollow shell by means of valve 20.

After the desired amount of foamable composition 14 has been injectedinto hollow shell 10, the shell is placed in an alternating magneticfield, such as that established by coils 22, as shown in FIG. 3. Whenthe induction coils are energized, the high frequency magnetic fieldestablished by the work coil penetrates throughout the chemical mixtureand creates a thermo-hysteretic effect in the susceptor particles. Eachparticle becomes heated on an individual basis and this heat immediatelybeing conducted into the contingent and surrounding portions of foamablecomposition 14.

The magnetic field has a negligible or zero effect directly on theintermediate chemical composition itself in the normal case since thereis no effective mechanism by which heat is generated in such organicmaterial by means of a magnetic field. Thus the main volume of thechemical composition remains cool while sufiicient local temperaturerises occur at each particle to initiate local release of gas causingfoaming and forming the foamed core for article 10, as shown in FIG. 4.When the magnetic field intensity is high enough, it is believed thatconsiderable foam development can occur in this manner with relativelylittle heat development in the general body of the foaming chemical. Theunique property of localized heat generation of the present inventionwill be found to offer possibilities for entire new classes of foamingmaterials that could not be used previously because of ex cessive heatdamage. Thus, it can be seen that this unique attribute differsdrastically from both the microwave or dielectric method and theconventional oven method in which no choice exists but to develop heatthroughout the entire mass.

Further, as the foaming proceeds, the relative density of the susceptorparticles decreases due to the expansion of, the chemical composition.This tends to decrease the heat generated per unit of volume providing ameasure of self regulation to the heating process which avoids heatdegradation or damage to the chemical composition.

Additional improvements and advantages result from the uniquecharacteristics of the induction foaming method of the presentinvention. When the attempt is made to apply dielectric and microwaveapproaches of past methods to foaming of mixtures that contain a certainamount of ionized liquid, it is found that the electrically conductivequality of the liquid can in effect short circuit the applied electricfield resulting in seriously reduced heating rate in the liquid. Withthe method of the present invention, on the other hand, those skilled inthe art will recognize that the liquid mass to be foamed can actuallybecome quite conductive electrically without seriously hampering thepenetration of the magnetic field of the present approach.

The frequency of the alternating magnetic field may be selected inaccordance with desired heating rates, type of susceptor utilized, etc.The frequency may typically be from .4 to 5000 megahertz. Typically, thefrequency range ofthe field will range from 2 megahertz to 30 megahertzfor the conventional helix-type of induction coil. It is desirable forthe magnetic field strength to exceed 50 oersteds and preferably toexceed 100 oersteds. Coils 22 may be cooled by coolant circulated inpassages 24, to prevent any possible scorching or degradation of hollowshell 10. After foaming has been effected, shell 10 may be removed fromcoils 22 and the fabrication of the foam core article is complete.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

We claim: 1. A method of producing a foam core article comprising thesteps of:

forming a hollow shell of a material capable of at least partiallytransmitting an alternating magnetic field, said shell forming a portionof the finished article and being further capable of containing a liquidand resisting forces generated by foaming Within the shell;

dispersing, in a foamable liquid capable upon heating of forming anexpanded foam composition a particulate susceptor in a size range offrom .01 to 20 microns and consisting essentially of electricallynonconductive metallic oxides having thermo-hysteresis properties, saidparticulate susceptor being incorporated in the liquid in quantitiessufficient to heat the liquid to a foamable condition;

injecting the liquid carrying the particulate susceptor into the hollowshell; and

exposing the liquid carrying the particulate susceptor to an alternatingmagnetic field of sufiicient strength within a frequency range of .4 to5000 megahertz to heat the particulate susceptor and the liquid to forman expanded foam composition within the hollow shell.

2. The method according to claim 1 wherein said liquid is foamable byinternal gas production and said method is further characterized assubjecting the liquid carrying the particulate susceptor to analternating magnetic field for generating the internal gas-producingheat.

3. The method according to claim 1 wherein the step of dispersing theparticulate susceptor is further defined as dispersing a susceptorselected from a class consisting of gamma Fe O and Pe oin said liquid.

4. The method according to claim 1 wherein the step of dispersing theparticulate susceptor is further defined as dispersing 5 to 50%particles by weight with respect to the foamable liquid.

5. The method according to claim 1 wherein the exposing step is furtherdefined as exposing the liquid to an alternating magnetic field having afrequency of between 2 and 30 megahertz.

6. The method according to claim 1 wherein the exposing step is furtherdefined as exposing the liquid to an alternating magnetic field havingan intensity in excess of 50 oersteds.

7. The method according to claim 6 wherein the exposing step is furtherdefined as exposing the liquid to an alternating magnetic field havingan intensity in excess of oersteds.

References Cited UNITED UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. I 3, 81.6, 574 Dated June '11., 1974 Inventor(s)William C Heller, Jr et al It is certified that error appears in theabove-identified patent and that said Letters Patent are herebyeorrected as shown below:

Col. 1 line 47 Delete "mahy" and Substitute therefor --may--- Sig nodandv sealed this 3rd day of December 1974.

(SEAL) Attestr v McCOY M. GIBSON JR. C. MARSHALL DANN' Attesting OfficerCommissioner of Patents UNITED STATES PATENT OFFICE @ERTIFICATE' OFCORRECTION June ll, 1974 Paten t No. 3, 816, 5574 Dated Inven-tor(s)William C Heller, Jr., et a1 It is certified that error appears in theabove-identified patent and that said Letters Patent are herebyCorrected as shown below:

Delete "many" and substitute Col. 1 line 47 1 therefor ---may-- Signedand sealed this 3rd day of December 1974.

(SEAL) Attest McCOY M. GIBSON JR. C. MARSHALL DANN' Attesting OfficerCommissioner of Patents USCOMMDC 60376-1 69 FORM PO-IOSO (10-69) l w u.so covimmwr mu'rmc OFFICI nu o-au-su,

